A Distributed Query Engine for XML-QL

This paper describes a distributed Query Engine for executing queries on XML documents. It supports complex pattern matching and a single pattern can be used for not only expressing selections and projection but also for a joins of one part of the document with another parts of the same document. The patterns can contain path expressions and tag variables as well. It is a multithreaded pipelined query engine with each operator running as a separate thread. A single query engine can execute multiple queries at the same time and execution of a single query can be distributed across different servers. The distributed servers are totally symmetric and queries can be submitted to any server. The server conforms to XML and proposed XML-QL standards and all communication among servers follow these specifications. This makes sure that even the query engines from other vendors can participate in a distributed query execution.